I’ve had similar experiences. See this:
There is something that changes within Roon as before and after the first album for an artist is added to the library. And it is not consistent or it would be easy to reproduce. Roon seems stubbornly unaware of some artists in search until you find them, add an album by the artist, and then Roon pops all kinds of better search results. Not just the one album you added, but it is almost like you have to prove to Roon search that an artist exists and there is content in your streaming service for that artist, and then once proven you get all kinds of quality hits.
I’ve experienced it a few times but only noticed the pattern after the fact, so it’s been hard to document precisely or trace each step. But definitely something is unusual with the interaction between search and the streaming services relative to artists that don’t have any content in your library.